Monoclinic gallium oxide (β-Ga₂O₃) nanowires (NWs) were synthesized via a vapor–liquid–solid mechanism by heating a GaN/sapphire template. The average diameter and length of the β-Ga₂O₃ NWs were around 100 nm and 10 μm, respectively. It was found that the conductivity of the β-Ga₂O₃ NW humidity sensors increased monotonically when the relative humidity (RH) was increased from 30% to 95%. To determine the relationship between ultraviolet (UV) light and humidity sensing properties, the humidity was measured with and without UV illumination, and the UV photoresponse was measured at various RH values, respectively. With UV illumination, the water molecules captured the electrons and holes generated by UV light in a high-RH environment, thus resulting in lower humidity sensitivity than that in the dark. The β-Ga₂O₃ NW UV photoresponse results indicate that the dissociated ions of water at high RH decrease the photoresponse of the NWs.